The present invention relates to a paper transporting device for an image recorder and, more particularly, to a paper transporting device of the type causing a toner image to be transferred to a paper sheet in an upper portion of a photoconductive element of an image recorder and then transporting the paper sheet face down to a fixing unit.
In a laser printer, electrophotographic copier, facsimile machine or similar image recorder implemented with a dry process electrophotographic procedure, a toner image formed on a photoconductive element is transferred to a paper sheet, and then the paper sheet is separated from the photoconductive element. The paper sheet carrying the toner image thereon is transported to a fixing unit for fixing the toner image. With a laser printer which belongs to a family of image recorders of the type described, it is not necessary to lay an original document on a glass platen for reproduction. Hence, an optical writing station for writing data in a photoconductive element can be readily provided below the photoconductive element, in turn allowing a toner image to be transferred to a paper sheet in a position above the photoconductive element. When imagewise transfer is performed over a photoconductive element as stated, it is possible to provide a linear paper transport path in the image recorder and to stack paper sheets face down one upon another, i.e., in order of page. Further, such a configuration allows a jamming sheet to be removed with ease and eliminates an occurrence that toner particles drop onto a paper sheet. With these advantages in view, there has been proposed a paper transport system of the type transporting a paper sheet face down above a photoconductive element, allowing a toner image to be transferred to the paper sheet in a position above the photoconductive element, and then transporting the paper sheet face down to a fixing unit.
The above-described type of paper transport system, however, has some problems left unsolved. Specifically, the photoconductive element is spaced apart from the fixing unit by a certain distance so as to protect the former against heat which the latter generates. This, coupled with the fact that a toner image is simply electrostatically deposited on the underside of a paper sheet before the latter reaches the fixing unit, it is not desirable to guide a paper sheet by a guide member of the kind making direct contact with the underside of a paper sheet. A sufficiently thick or sufficiently elastic paper sheet would be successfully transported as far as the fixing unit without the need for such a guide member. However, a majority of paper sheets usable with an image recorder does not have such a thickness or elasticity. Specifically, a paper sheet is usually sustained in a cantilever fashion before its leading edge reaches a fixing roller of the fixing unit or after its trailing edge has been separated from the photoconductive element. In this condition, an ordinary paper sheet necessarily bends downward due to gravity resulting in the leading edge thereof missing the fixing roller or in an image portion adjacent to the trailing edge rubbing against another member. In the light of this, a number of holes may be provided above a paper transport path in order to cause a fan to suck a paper sheet from above during the transport of the paper sheet, as already proposed in the art.
However, a prior art paper transporting device adopting the above-mentioned suction scheme is not satisfactory for the following reasons. Since a paper sheet is transported while being sucked from above through the number of holes, once it stops up the holes with its upper surface, not only the load acting on the fan but also the intensity of vacuum are increased. The intensified vacuum sucks the paper sheet strongly against the holes and thereby increases the resistance to the sliding movement of the paper sheet on and along a guide member which is formed with the holes. Furthermore, the holes are apt to catch the corners of the leading edge of a paper sheet having a particular size, bending them or causing the paper sheet to jam the transport path.